A degree of integration in a semiconductor device, particularly in an integrated circuit using a MOS transistor, has been increasing year by year. Along with the increase in the degree of integration, miniaturization of the MOS transistor used therein has progressed to a nano region. The progress in miniaturization of the MOS transistor, which constitutes an inverter circuit as a basic circuit for digital circuits, gives rise to a problem, such as difficulty in suppressing a leak current, which causes deterioration in reliability due to hot carrier effects and poses an impediment to sufficiently reducing a circuit occupancy area while meeting a requirement of ensuring a necessary current magnitude. With a view to solving this problem, there have been proposed a surrounding gate transistor (SGT) having a structure in which a source, a gate and a drain are arranged in a direction perpendicular to a substrate, wherein the gate is formed to surround an island-shaped semiconductor layer, and a CMOS inverter circuit using the SGT (see, for example, S. Watanabe, K. Tsuchida, D. Takashima, Y. Oowaki, A. Nitayama, K. Hieda, H. Takato, K. Sunouchi, F. Horiguchi, K. Ohuchi, F. Masuoka, H. Hara, “A Novel Circuit Technology with Surrounding Gate Transistors (SGT's) for Ultra High Density DRAM's,” IEEE JSSC, Vol. 30, No. 9, 1995).
An inverter is constructed using a pMOS transistor and an nMOS transistor. In an inverter circuit, a gate width of a pMOS transistor has to be set to be twice as large as that of an nMOS transistor, because a hole mobility is one-half of an electron mobility. Therefore, a conventional CMOS inverter circuit using an SGT (SGT CMOS inverter circuit) is composed of two pMOS SGTs and one nMOS SGT. In other words, the conventional SGT CMOS inverter circuit is composed of total three island-shaped semiconductors.
An SRAM is composed of two inverters and two selection transistors. In cases where an SRAM is constructed using the conventional SGT CMOS inverter circuit, it is composed of four pMOS and four nMOS. In other words, the SRAM using the conventional SGT CMOS inverter circuit is composed of total eight island-shaped semiconductors.